Having done something very similar to this, I would like to weigh in. First, it looks like your frame is is good shape. What I see is rust on the underside of your tub (body). Your body mounts look good as well. Have you tried to loosen any of your body mount bolts yet? This could give you an indication as to any rust you may have inside the tub, where the bolts thread into.
At the least I would poke around and chip away all of the scale and rust to get the best look at what you are dealing with. If you're lucky, you can weld a plate over that section, and call it a day. Good luck.